The Electronic Frontier Foundation EFF has produced a Legal Guide for Bloggers. Of course, this is not The Law. It is presented as a guide, and it addresses the issues that bloggers may face and what recourse they may have if they run into problems.
If you read through all the FAQ's, you'll find a very comprehensive overview about guidelines and how to avoid stepping into legal puddles.
Have any of you had to face any legal battles with statements you've made in your blogs so far?